If you have decided to have two hard disks on your computer then:
Set the bootable as primary master and the other one as the secondary master.
Set your CD-ROMs as the secondary slave. CD-ROMs usually have four settings: Master, Slave, Cable Select, and Single Drive.
Here, the single drive is used only if it is the only device on the specific IDE channel. Therefore if you intend to have only one hard drive on a primary channel, you use single drive. 
